A draw-down jaw known from German Patent Specification No. 910,280 has a bearing jaw and a clamping jaw, an inclined plane being provided between bearing jaw and clamping jaw which is designed and arranged in such a way that the clamping jaws acting on the workpiece with parallel and vertically arranged gripping surfaces execute a certain motion downwards in the direction of the bearing surface for the workpiece when the gripping force is applied, and thereby bring about the downward draw. During this operation the clamping jaws are displaced obliquely downwards parallel to themselves. In conjunction with the inclined plane, restoring springs are provided which guide the clamping jaws back into their initial position on the bearing jaws when the gripping force is removed. These restoring springs are often dimensioned comparatively weakly. In order to be able to use strong spring forces, the restoring of the clamping jaws is limited by a stop. In the machining of a workpiece in the vise, coolants are often used, which then also flow off over the vise and the gripping jaw as well as the bearing jaw. These coolants tend to make the inclined plane stick, so that the restoring springs are no longer able to bring about the initial position. In such a state, the work piece can only continue to be gripped horizontally and there is no downward draw. For proper functioning, it is therefore necessary to keep the inclined plane between bearing jaw and clamping jaw clean and to oil it. Good functioning therefore requires appropriate maintenance. The arrangement of a seal covering the guide surfaces at the inclined plane also counteracts these disadvantages. Under high gripping pressures, the frame of the vise distorts in the elastic range. As a result, the parallelism of the gripping surfaces of the clamping jaws is lost and the workpiece is horizontally gripped relatively low down, that is in the region of the bearing surface, while in the upper region it is held little or not at all. In this case as well, the downward draw is completely or partly lost.
For the gripping of workpieces of irregular surface, in particular in the case of a trapezoidal cross-section, it is known to use swing jaws. In this arrangement, a plane clamping jaw is suspended so as swing about an axis on the vise, i.e. it rests against the workpiece according to its oblique surface during the gripping operation. As a rule, only a horizontally acting gripping force comes to bear in this case. A downward draw is not intended and, if at all, comes about coincidentally if the workpiece surface happens to be shaped accordingly.
Furthermore, roller draw-down jaws are known which have a two-part jaw. The part of the jaw which faces away from the workpiece bears an inclined plane. The part of the jaw which faces toward the workpiece is equipped with a roller mounted on it which acts simultaneously on the workpiece and on the inclined plane. Such roller draw-down jaws serve for the gripping of workpieces having angle errors. During the gripping operation, the roller rolls downwards on the inclined plane and at the same time on the workpiece. Since the direction of rotation of the roller is predetermined by the inclined plane, a corresponding downward draw is also achieved.
These known gripping jaws may either be fitted directly when equipping a vise, and thus be used with it. But it is also possible to design these jaws in such a way that they can be subsequently fitted in a vise with gripping jaws present there, or it can be hooked thereupon.